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Anyway I can use two or more SPST relays function as a SPDT relay? Answered

I have got many SPST relays but no SPDT ones. The project that I am making requires SPDT relays, so is there anyway I can connect two or more SPST relays that could possibly function as a SPDT relay, by that I mean, a SPST relay turns on another circuit when the coil is energized, but is there anyway I could use more of them so that if the coil of any one or more is energized, along with completing another circuit, it also simultaneously breaks another and vice versa.


Depending on how much current the coil requires, you can probably use a pull up resistor on the coil of one relay so that there is normally current flowing through it, then have a second relay to connect the coil to ground when power is applied. This will however require a fairly high power resistor in order to withstand dropping the full supply voltage.

Alternatively, a NOT gate and a large transistor or MOSFET can be used to mare efficiently turn off a relay when a signal is applied.

I will use it in a mobile robot, how will I provide earth there?

In that case ground is the 0V rail in the circuit, the negative terminal of the battery, and usually the chassis of the robot.

I don't understand, I don't know much about electronics so can you please explain again in a bit more detail that how will one relay turn off another relay when voltage is applied on it?

And I don't know what is a pull up resistor.

One side of the coil is connected to ground, the other is connected to your power supply through a resistor. The resistor allows enough current to flow through it to make the coil work, but won't short out the battery if the coil is shorted out.

The second relay is connected across the coil of the first, so that when it is turned on, it shorts out the coil of the first relay. Because the coil is shorted, no current flows through it, so it turns off when power is applied to the second relay.

You need to select the large enough resistor that will still allow enough current and voltage pass to the coil of the first relay for it to turn on.

Will this work,

The first relay is directly connected to the battery, not through any resistor, but when the second relay turns on, it connects the resistor in parallel with the first relay. Will the relay turn off?

Connecting a resistor in parallel with the coil will not do anything, unless it is small enough to draw way too much current from the battery.

Yes. I thought of this very same thing first when I started wondering about this except that I hadn't placed the resistor and I thought this will short out the battery.